The present invention relates to an electrically power assisting steering apparatus.
In the operation of an electrically power assisting steering apparatus (hereinafter called as an EPS apparatus) wherein turning force of an electric motor becoming auxiliary steering torque is decelerated by a gear device to transmit it to an output shaft, thereby to assist a driver""s power for rotating a steering wheel to steer wheels, e.g., when the steering wheel is rotated up to a stroke end of a rack and the steering is abruptly stopped, the motor can not stop immediately due to inertia force and occasionally, excessive torque is produced in the decelerating gear portion. In order to protect decelerating gears against the excessive torque, conventionally, a torque limiter using a friction plate is provided between a motor and a worm, or a torque limiter using a ring member for applying elastic force is provided between an output shaft and a worm wheel. Thereby, at the time of generation of such excessive torque, slip is developed between both of them to prevent transmission of the excessive torque.
In an EPS apparatus, generally, a worm is formed of iron material, and a worm wheel is made by forming a gear portion of resin material integrally on the outer peripheral surface of a core portion of solid iron material by way of adhesion, fusion or injection molding. Also, an output shaft having the worm wheel mounted thereon is formed of iron material the same as the core metal.
In this field of the art, the ring member for applying elastic force used for the torque limiter provided between the output shaft and the worm wheel is called as a rotation transmitting member or a torque setting member, etc., or is known as a tolerance ring (trade name: Rencol Tolerance Rings).
As mentioned above, in the conventional EPS apparatus, since the core metal portion of the worm wheel constituting the gear device is formed of solid iron material, there are the following problems. First, when attempting to correspond to achieving high output of the EPS apparatus, it is necessary to make the diameter of the worm wheel large for high strength and high module ability. However, in that case, the weight of the core metal portion is increased and inertial force in steering is increased, so that a driver""s feeling in rotating the steering wheel and cutting of the steering wheel are deteriorated. Also, in compliance with the enlargement of the diameter of the core metal portion, when the diameter of the resin gear portion is enlarged, accuracy of the gears might be worsened to cause increase of noise of the gears in operation since dimensional change of resin due to moisture absorption and temperature is large. Also, the strength of the gear might be lowered since there is the danger of occurring voids in the resin portion due to injection molding. Thus, in the conventional EPS apparatus, there are problems in making the diameter of the worm wheel large.
Next, by comparison on the basis of ordinary temperature, the resin gear portion has characteristics in that its strength is lowered under high temperature and is heightened under low temperature. However, the conventional output shaft and the core metal portion are made of iron material and their linear expansion coefficients are approximately the same, so that the limit torque of the torque limiter is constant to temperature. Therefore, the limit torque of the torque limiter needs to be set at the maximum operating temperature, and needs to be set low, whereby the difference between the limit torque and the transmission torque was small. When the ring is worn and the limit torque is lowered because of long-term use of the EPS apparatus, the torque transmission might not be carried out, and thereby the designing is difficult.
For example, a worm wheel is disclosed in the Japanese Utility Model Publication No. 2556890 and the Japanese Patent Application Laid-Open No. 7-215227. As shown in FIGS. 15A and 15B, the worm wheel has a core metal X19 and a resin ring X20. The entire outer peripheral surface of the core metal 19X is formed in the shape of a gear with irregularities X19a constituted of rotational direction stopping serrations X19b and axial direction stopping projections X19c. The resin ring X20 with teeth on its outer peripheral surface is fitted on the irregularities X19a of the core metal X19. As the thickness of the resin ring X20 is properly set, it is possible to maintain proper backlash between the gears even though atmospheric temperature is changed.
Also, in a worm wheel disclosed in the Japanese Patent Application Laid-Open No. 11-192955, an approximately gear-shaped core metal is used for enhancement of the strength of the teeth, and the gear-shaped rim portion of the core metal is covered by a resin by way of injection such that the cross section of the resin in the axial direction becomes a rectangle. Then, the binding of the rim portion and the resin depends on the strength of the cooled and hardened thin resin layer.
However, in the conventional worm wheel of the Japanese Utility Model Publication No. 2556890 and the Japanese Patent Application Laid-Open No. 7-215227, the resin ring X20 is fitted on the irregularities X19a formed on the outer peripheral surface of the metallic core metal 19X, but since the mechanical strength of the resin is inferior to that of metal, the module needs to be enlarged in comparison with a metal gear, and the thickness of the teeth needs to be made larger than that of the metallic portion, whereby there occurs a problem that the decelerating mechanism is enlarged.
Also, the resin with the large thickness is inferior in heat-dissipation ability, and so the thickness of the resin is set appropriately, but there is a problem in that wear of the resin is increased due to heat produced in the engagement of the gears. Especially, as shown in FIG. 16, when the worm wheel is incorporated in the EPS apparatus and disposed in an engine room, the temperature condition is severe and it is difficult to secure the durability under high temperature environment. (Description of the structure of an EPS apparatus in FIG. 16 will be made later in the description of the preferred embodiments and then omitted here).
Further, since the binding power of the core metal X19 and the resin ring X20 depends on mechanical catching of the irregularities 19a, it is necessary to form the thickness of the resin ring X20 large so as to make the resin ring X20 withstand the slipping-off force. In case of using nylon family resin, as it has water absorptive property, the larger the volume of the resin becomes, the larger its dimensional change caused by expansion due to water absorption becomes. However, in the EPS apparatus, as a driver rotates a steering wheel, a steering assisting force is transmitted via the decelerating mechanism, whereby the rotational directions in the decelerating mechanism are reversed intricately. Therefore, in order to prevent noise from the teeth surfaces of the gears, it is necessary to control such that the backlash becomes extremely small. As mentioned above, when the volume of the resin is large, its dimensional change due to water absorption is considerably large. Therefore, there is a problem in that when the resin absorbs water and expands to the extent that the backlash is lost, the operability is deteriorated.
Further, in the worm wheel disclosed in the Japanese Patent Application Laid-Open No. 11-192955, external force generated due to the engagement of the gears acts on the thin resin layer as tensile stress, so that the resin is easily broken and it is difficult to obtain strength. Also, in a case where the core metal is used as the insert in an injection molding, when the high temperature resin is cooled and contracted, residual tensile stress is produced. In this case, the joining of the resin and the core metal relies on mechanical catching, and if only a one portion of the resin is broken, the broken-out surface is enlarged due to the internal stress, so that the binding force is lost.
Two types of resin worm wheels are mass-produced: resin of MC nylon type and resin containing glass fiber type. In the case of the former MC nylon worm wheel, while a tube-like MC nylon and a core metal with irregularities (knurls) as described above on its outer peripheral surface are subjected to high frequency induction heating, their joining portions are tight-contacted and bonded with adhesive. However, the cost of the MC nylon is very high, and its heat resistance is up to 80xc2x0 C.
On the other hand, the latter worm wheel is produced by injection-molding the resin on irregularities of the outer peripheral surface of a core metal which is fairly lager than that of the MC nylon type, and thereby fixing the resin by the irregularities mechanically. In this case, since heated resin is injected, the resin is contracted when cooled, and tensile stress acts on the interface between the resin and the core metal. Accordingly, if even a one portion of voids and a weld line is broken, a crack is spread and the fixing force between the core metal and the resin is lost. Further, although the heat resistance of the resin is up to 120xc2x0 C., there is a problem in that operability is reduced due to expansion of the resin caused by water absorption.
It is a first object of the present invention to provide an electric power steering apparatus equipped with a power transmission mechanism of an electric driving section capable of corresponding to high output, and achieving predetermined slip torque in accordance with change in operating temperature.
In order to achieve the first object of the present invention, in an EPS apparatus provided with a torque limiter having a ring member for applying elastic force between a worm wheel and an output shaft, the output shaft is made of iron material, a gear portion of the worm wheel is made of resin material, and a core metal portion thereof is made of metallic material whose specific gravity is small and linear expansion coefficient is large with respect to the iron material thereby to set limit torque of the torque limiter low under high temperature and high under low temperature.
It is a second object of the present invention to provide an EPS apparatus having a worm wheel incorporated in a transmission pathway of auxiliary steering force in which a core metal and a resin are bonded with strong bonding power, heat dissipation ability is preferable, and miniaturization is possible by the use of the minimum amount of resin.
In order to achieve the second object, in an EPS apparatus having a worm wheel incorporated therein for transmitting driving force of a motor as auxiliary steering force together with a worm gear, the present invention proposes the worm wheel formed by joining a thin resin to the entire outer peripheral surface of teeth portion of a gear-shaped core metal by way of chemical bond according to composite molding technique, or by means of adhesive. Further, the core metal can be made of aluminum alloy or copper alloy. As structured above, the bonding of the teeth portion of the core metal and the resin becomes strong and even though the thickness of the resin is made small, it is hard to receive tensile stress due to the engagement of the gears. Therefore, the resin is prevented from breaking and the amount of resin can be reduced, whereby dimensional change of the gear due to water absorption can be reduced considerably. Since the core metal is made of aluminum alloy or copper alloy, heat is easily diffused and it can be used under comparatively high temperature. Especially, when the core metal is made of aluminum alloy and incorporated in the EPS apparatus as the worm wheel, it has the same thermal expansion coefficient as the aluminum die-cast gear housing, change of backlash of the gears due to thermal expansion is reduced extremely, making it possible to maintain preferable operation of the gears constantly.